1. Field of the Invention
The present invention relates to a process for making polymeric films, material packaging and shaped articles like gasoline tanks, that meet specific technical requirements such as barrier and mechanical properties while being easy and cheap to manufacture by extrusion.
2. Brief Description of the Prior Art
Traditionally, polymer films or shaped materials are of a multilayer structure and are produced by coextrusion of a polyolefin and of a condensation polymer incompatible with the polyolefin.
The multilayer structure of the coextruded product is achieved by combining different layers in a die before their extrusion as a preform. The obtained preform is then blown and molded in the form of the final product.
Polymer blending by extrusion followed by stretching at the exit of the die was found to impart the obtained films with enhanced barrier properties. Drawing orients the matrix microstructure and the particles of the dispersed phase in the direction of drawing.
Kamal et al. in U.S. Pat. No. 5,188,784 (1993) disclose a special sophisticated die system allowing to achieved such a post extrusion orientation.
Faisant and al. in Polymer, Volume 39, No. 3, 1998 disclose the extrusion of blends of ethylene vinyl alcohol copolymer (EVOH) dispersed in either polypropylene or in polyethylene. The so obtained film is then stretched through a flat die. Such a drawing is disclosed as inducing a mixture of lamellae and fibrils of EVOH in polyolefin matrix. The polyolefin/EVOH blends thereby prepared are compatibilized using commercial maleic anhydride functionalized polyolefins. This approach is found to lead to materials with 85% decrease in the permeability to oxygen in comparison with that of pure polyolefin.
Polymer blends used in such processes include two or more polymers that are mixed physically. Compatible blends yield polymer alloys, whereas most of the commercial blends comprise incompatible polymers to form a dispersion of one polymer in the other one. The behavior of polymer blend product depends to a large extent, on the microstructure of the blend reflecting the size and distribution of the dispersed phase, and the nature of the interface between the two phases.
P. Subramanian in U.S. Pat. No. 4,410,482 (1983) discloses a method for producing a lamellar structure from a polymer blend. In a first step, an heterogeneous blend of polyolefin, a condensation polymer incompatible with the polyolefin and a compatibilizer consisting of an alkylcarboxyl-substituted polyolefin, is prepared. The heterogeneous blend is heated above the highest melting point of the blend constituents. The body of the molten blend obtained by extrusion is elongated by means of a conventional equipment.
The polyolefin forms a continuous matrix phase while the condensation polymer incompatible with the polyolefin presents the form of a discontinuous distribution of the thin, substantially two-dimensional, parallel and over-lapping layers. The compatibilizer, namely the alkylcarboxyl-substituted polyolefin, is present between the matrix phase and the layers of condensation polymer to adhere these matrix and layer together.
To carry out such multilayer extrusion process, the required equipment is complex and expensive, and at least one adhesive tie-layer is necessary.
The numerous works published on the matter stress the difficulty to design a low-cost material for use as packaging or for the manufacture of gasoline tanks, that have barrier and mechanical properties sufficient to make them efficient.
Such a material must be easy to process at low cost and due to recent environmental laws, it has to be recyclable. Optical clarity, in the case of films for food packaging, is also often required.
Plastic containers with high barrier properties are usually made of a multi-layers material produced by coextrusion. This is a complex and expensive technology and the final product is not recyclable. Therefore, polymer blending appears to be a more beneficial alternative in designing materials having enhanced physical properties with the possibility of recycling the final product. The addition of a small quantity of a barrier material into a low-cost matrix material can lead to a low-cost product with greatly improved barrier properties. Polypropylene (PP) is a suitable polymer with good mechanical and good barrier properties to H2O. On the other hand, EVOH which, as aforesaid, is a copolymer of ethylene and vinyl alcohol, has high barrier properties to gases such as O2 and CO2 and a high resistance to hydrocarbons. This makes EVOH an interesting candidate for food applications. However, EVOH is an expensive material.
There is therefore a need for a process allowing the preparation of films with a reduced amount of EVOH.
A first object of the invention is to provide a process for making a multiphase polymeric film having a lamellar structure with controlled permeability and/or controlled mechanical properties.
A second object of the invention is to provide the multiphase polymeric film with a lamellar structure obtained by the said process for making a multiphase polymeric film having a lamellar structure with controlled permeability and/or controlled mechanical properties.
A third object of the invention is to provide granules having a lamellar structure, which are obtained by grinding the multiphase polymeric film with lamellar structure obtained by the said process for making a multiphase polymeric film having a lamellar structure with controlled permeability and/or controlled mechanical properties.
A fourth object of the invention is to provide a process for producing a shaped article of polymeric material by using the granules obtained by grinding the multiphase polymeric film according to the invention.
A fifth object of the present invention is to provide a shaped article with improved permeability to oxygen and improved mechanical properties.
The process according to the invention for making the multiphase polymeric film as broadly disclosed hereinafter comprises a step of:
preparing a molten blend made of
a first polymer phase dispersed in a second polymer phase which is a matrix polymer phase incompatible with the said first phase and
a compatibilizer selected from the group consisting of DEM, MAH, DEM-g-SEBS, MAH-g-SEBS, DEM-g-PP and MAH-g-PP;
extruding the molten blend through a flat die provided with an exit
stretching the so-extruded blend downwards said exit at a preselected stretching ratio to produce the said multiphase polymeric film, and
solidifying the extruded film sufficiently rapidly to preserve the lamellar structure.
The invention as claimed is however restricted to the used of DEM, DEM-g-SEBS and DEM-g-DP as compatibilizers.
Granules with a lamellar structure according to the invention, are obtained by grinding the so obtained multiphase polymeric film.
These granules can be used for producing shaped articles of polymeric material.
The objects, advantages and other features of the present invention will become more apparent upon reading of the following non restrictive description of a preferred embodiment thereof, given by way of example only with reference to the accompanying drawings.